a) Field of the Invention
The present invention relates to an observation optical system which permits observing, through an eyepiece lens, an image of object which is formed by inverting, with a relay lens, an inverted image formed by an objective lens.
b) Description of the Prior Art:
In the conventional viewfinder optical system for single-lens reflex cameras illustrated in FIG. 1, a light bundle having passed through a photographic lens is split by a quick return mirror 5 into two light bundles to be used in a photographic system and a viewfinder system respectively. The light bundle to be used by the viewfinder system forms an inverted image on a screen mat 6, which is inverted to form an erect image by a pentagonal prism 7 for observation through an eyepiece lens 4.
On the other hand, electronic still cameras and video cameras use, as image pickup elements, 2/3 inch and 1/2 inch CCD image sensors and so on which are too small for cameras designed for use with a 35 mm film. Accordingly, the eyepiece lens systems which are similar to those used in the conventional cameras for the 35 mm film do not permit observing images of sufficiently large sizes. For this reason, it is necessary for electronic still cameras and video cameras to enhance magnification levels of the eyepiece lens systems by shortening the focal lengths thereof. When the focal lengths of the eyepiece lens systems are shortened for enhancing the magnification levels thereof, however, the distances as measured from the eyepiece lens systems to the surfaces of images to be observed are shortened. In order to arrange erection prisms, etc., it is necessary to select a compositions of the eyepiece lens systems in which the principal points of the eyepiece lens systems are shifted toward the surfaces to be observed. That is to say, the eyepiece lens systems must have, when they a reconsidered as photographic lens systems, long back focal lengths.
As the conventional examples of viewfinder optical systems to be used with the single-lens reflex type electronic still cameras and video cameras, there are known the optical systems disclosed by Japanese Patent Kokai Publication No. Sho 61-29816 and Japanese Patent Kokai Publication No. Sho 60-233628.
The former example uses a roof prism, as the erect prism, in combination with an eyepiece lens system composed of five or six lens elements. The latter example uses a trigonal prism, as the erect prism, in combination with an eyepiece lens system composed of three or four lens elements.
These conventional examples require high manufacturing costs since the eyepiece lens systems thereof are composed of large numbers of lens elements as described above and the roof prism must be polished with high precision when the eyepiece lens system is to have a high magnification level.
Further, as a method to convert an inverted image into an erect image, there is known the method disclosed by Japanese Patent Kokai Publication No. Hei 1-222214. This method uses, in place of the roof prism, a relay lens system which inverts and refocuses an image formed by a photographic lens system. The patent also discloses a viewfinder optical system which permits observing the image inverted by the relay lens system through an eyepiece lens system. The relay lens system and the eyepiece lens system are composed of six lens elements, and the viewfinder optical system has a distance of approximately 120 mm as measured from the eyepoint to the image surface thereof and a magnification of .times.13.5.
In the conventional example described above or a viewfinder optical system which is generally referred to as the real image type, both the relay lens system and the eyepiece lens system have positive refractive powers, and Petzvals's sum of the viewfinder optical system has a large positive value which causes degradation in image quality. Especially when the viewfinder optical system comprises a larger number of lens elements or has an enhanced magnification, the Petzval's sum is further aggravated and cannot be corrected.